1. Field of the Invention
The present invention relates to a vulcanizing mold for producing pneumatic tires.
2. Description of Related Art
Vulcanizing mold for pneumatic tires has inner surface that is provided with ridges for defining the tread pattern of the product tire. Thus, air tends to be trapped between the inner surface of the mold and the outer surface of a green tire upon vulcanization thereof. In particular, the ridges of the mold and the unvulcanized rubber of the green tire are brought into tight contact with the mold surface and thereby prevent effective discharge of air from the mold cavity. As a result, air trapped between the mold surface and the rubber surface forms voids or the like surface defects (so-called “bares”) in the outer surface of the product tires, thereby lowering the commercial value of the product tire.
Therefore, it has been a conventional practice to provide a mold with vent holes in the form of fine apertures, for discharging air out of the mold cavity. In this instance, however, rubber tends to be extruded out of the vent holes to form spews on the tire surface, which degrade the appearance of the product tire. Thus, in order to provide an improved appearance of the product tire, time-consuming and/or troublesome countermeasures are often required. Such countermeasures include, for example, trimming of spews, and/or formation of increased number of fine apertures in the mold.
In view of these problems, JP-A-2001-18235 discloses a segmented-type vulcanizing mold including a plurality of mold pieces for forming a tread portion of the tire, which are assembled with each other in the circumferential direction of the tire. Each mold piece has a molding surface provided with various ribs for forming a tread pattern of the tire, and end surfaces where adjacent mold pieces are in abutment with each other when they are assembled. The end surface of the mold piece includes an edge region adjacent to the molding surface and extending in a width direction of the tread portion. The mold piece has a vent passage communicating with atmosphere, formed of a narrow gap that extends along the edge region partly in the width direction of the tread portion.
The vulcanizing mold disclosed in JP-A-2001-18235 proved to be highly advantageous in that the vent passage in the form of a narrow gaps along the edge region of each mold piece can be formed in a facilitated manner, more or less eliminating difficult and time-consuming machining of fine apertures in the mold pieces. However, even with the vulcanizing mold of such an improved structure, it is often necessary to apply vacuum in order to positively discharge air from the mold cavity and thereby effectively avoid formation of surface defects in the outer surface of the product tires. This leads to the necessity of providing vacuum pump device in combination with a complicated seal structure for the vulcanizing machine as a whole, besides increased equipment cost and running cost.